1. Technical Field of the Invention
The invention pertains generally to sample testing products and techniques, and more specifically to an improved solid phase extraction filtration disk and a method for its use.
2. Description of the Related Art
Hexane Extractable Material (HEM) is a method-defined analyte based on the procedures set forth in Environmental Protection Agency (“EPA”) Method 1664. This method is used in the EPA's survey and monitoring programs of surface and saline waters and industrial and domestic aqueous wastes under the Clean Water Act; the Resource Conservation and Recovery Act; the Comprehensive Environmental Response, Compensation, and Liability Act; and other EPA regulatory programs to determine the levels of n-hexane extractable material (oil and grease) and n-hexane extractable material that is not adsorbed by silica gel (non-polar material). Extractable materials include relatively non-volatile hydrocarbons, vegetable oils, animal fats, waxes, soaps, greases, and related materials.
EPA Method 1664 is based on liquid-liquid extraction. Simply, after extracting the oil and grease from the sampled water into a solvent, generally n-hexane, the n-hexane is evaporated and the total mass of material remaining is measured and reported as Hexane Extractable Material (HEM), or Silica Gel Treated Hexane Extractable Material (SGT-HEM). Differentiating HEM from SGT-HEM requires separate sample preparation by the technician. Further, the technician must ensure that there is no water present and all the n-hexane is evaporated, as the presence of either will result in over-reporting the HEM content of the sample. This means one measurement can take up to 48 hrs. Thus, EPA Method 1664 is both labor intensive and time consuming.
In general, modification of a method-defined analyte method is not allowed. EPA Method 1664, however, does list provisions allowing for some performance-based modifications, provided those changes do not degrade method performance or alter the chemistry of the method. In such a revision, the EPA allows solid phase extraction of the HEM from water using disks or cartridges, followed by the elution of the HEM from the disks or cartridges with n-hexane. The n-hexane is then evaporated from the sample and the remaining material is weighed to determine the HEM. SGT-HEM is determined by re-dissolving the HEM in n-hexane to perform the silica gel treatment. Thus, the revised EPA Method 1664 not only reduces the amount of solvent required, but also reduces the time required to perform the test.
Generally, the art of separation science, which involves extraction and chromatography, has two main objectives. One is high yield extraction and recovery of a targeted analyte and the other is a rapid rate of extraction and elution. A specific type of extraction used for separation is solid phase extraction, also known as SPE. SPE is a method of sample preparation that removes and concentrates an analyte from a liquid sample by absorption or adsorption onto a disposable solid phase medium. This is followed by elution of the analyte with a solvent appropriate for analysis.
SPE can be carried out using extraction disks which include particle loaded membranes of various diameters as the extraction medium. For example, one such disk comprises a membrane that includes sorbent particles (e.g., C-8 and C-18 bonded silica particles) immobilized in a web of poly(tetrafluoroethylene) (PTFE) microfibrils. Another exemplary disk includes a web of glass microfibers impregnated with chemically bonded silica sorbents such as C-18 aliphatics. For general use, these SPE disks can be supported on a glass or polymer frit in a standard filtration apparatus using vacuum to generate the desired flow of sample through the disk.
Currently, for SPE to work correctly in a method such as EPA Method 1664, the C-18 silica material in the extraction disk must go through a pre-conditioning process using a polar solvent such as methanol. Such disks are described in U.S. Pat. No. 5,391,298. The methods to fabricate existing extraction disks are also described in U.S. Pat. Nos. 5,538,634 and 6,475,340. However, the use of a polar solvent can lead to unacceptable modifications to EPA Method 1664. Thus, the technician must be extremely vigilant to remove all of the polar solvent from the extraction disk, yet keep the disk wetted, prior to performing the analytical method. That is, the disk must be pre-conditioned by solvent and remain wet before the addition of testing sample or the test may become out of compliance. While experienced technicians can master this technique and automated extraction systems provide more consistency and control, there is potential for error using such a pre-conditioning step.
To solve this problem, the Applicants have developed an SPE extraction disk, a testing system utilizing this extraction disk, and a method for testing under EPA Method 1664 that does not require pre-conditioning with a polar solvent. This eliminates any potential of a co-solvent coming into contact with the sample and non-compliance because of such.